Field of the Invention
The present invention is directed to mucolytic compounds that are more effective, and/or absorbed less rapidly from mucosal surfaces, and/or are better tolerated as compared to N-acetylcysteine (NAC) and DTT.
Description of the Background
Many modern drugs are discovered through high-throughput screening or combinatorial chemistry. These compounds often are selected for their high pharmacological efficacy but unintentionally have poor drug-like characteristics (e.g., solubility, bioavailability, stability). One strategy to overcome these physiochemical, biopharmaceutical, and pharmacokinetic limitations is to use a prodrug form of the compound, a molecule that is inactive until undergoing an enzymatic or chemical transformation in vivo. Depending on the type of modification, prodrugs can have key advantages over their active counterparts: 1) low/no odor until activated, 2) increased stability and shelf-life, 3) increased aqueous solubility, 4) improved bioavailability, 5) improved oral absorption, 6) increased lipophilicity/permeability, and 7) improved parenteral administration.
Of the drugs approved worldwide, 5-7% can be classified as prodrugs. These drugs are classified into two categories, bioprecurser prodrugs or carrier-linked prodrugs. Bioprecurser prodrugs are converted into pharmacologically active drugs by metabolic or chemical transformation. Carrier-linked prodrugs have a promoiety that is covalently linked to an active parent molecule. This promoiety is released, usually by enzymatic hydrolysis, activating the parent molecule once delivered to the therapeutic location. Design of the prodrug moiety is usually based on the drug-like characteristics that need improvement in a particular molecule, the available functional groups that are amenable to a promoiety, and the targeted organ or tissue. In cases where the promoiety cannot be directly attached due to reasons such as steric hinderance, spacers or linkers are also added. In order to be well-tolerated, the promoiety should be non-immunogenic, stable until reaching the therapeutic tissue, and rapidly excreted from the body, once cleaved from the parent. Esters are one of the most commonly used promoieties, due to their ease of removal from the parent drug by ubiquitous esterases (e.g., acetylcholinesterases, butyrylcholinesterases, carboxylesterases, arlesterases), capability of increasing drug solubility by masking charge groups, such as carboxylic acids and phophates, and relatively simple synthesis. Some other common functional groups that are utilized as promoieties are: carbonates, carbamates, amides, phosphates, and oximes.
Prodrugs could be particularly useful as inhaled therapeutics for muco-obstructive respiratory diseases, such as chronic bronchitis (CB), including the most common lethal genetic form of chronic bronchitis, cystic fibrosis (CF). In a normally functioning lung, the primary defense against chronic intrapulmonary airways infection (chronic bronchitis) is mediated by the continuous clearance of mucus from bronchial airway surfaces, removing potentially noxious toxins and pathogens from the lung. In a healthy lung, the airway surface liquid is primarily composed of salt and water in proportions similar to plasma (i.e., isotonic). Ion transport properties regulate the amount of salt and water, and goblet cells and glands control the quantity of mucins on the airway surface. Mucin macromolecules organize into a well-defined mucus layer, which traps inhaled bacteria and is transported out of the lung via the actions of cilia, which beat in a watery, low viscosity solution termed the periciliary liquid. When there is an imbalance of the mucin to liquid ratio, the mucus becomes excessively viscous and adherent, which can lead to airway mucus accumulation and infection because the cilia cannot beat to clear the mucus.
Recent data indicate that the basic defect in both CB and CF is the failure to clear mucus from airway surfaces. As described above, the failure to clear mucus reflects an imbalance between the amount of airway surface liquid and mucin on airway surfaces. Patients with mucus-obstructive diseases, including CF, CB associated with cigarette smoke exposure (i.e., COPD), and asthma, exhibit increases in mucus concentration, as quantified by % solids (FIG. 1), as a result of reduced airway hydration and mucin hypersecretion due to goblet cell and glandular hyperplasia. Both as a function of disease severity, and in acute exacerbations, raised mucin concentrations produce adherent mucus that sticks to epithelial cells, impairs clearance, and triggers inflammatory responses and airway wall injury. The reduction in mechanical clearance of mucus from the lung leads to chronic bacterial colonization of mucus adherent to airway surfaces. It is the chronic retention of bacteria, the failure of local antimicrobial substances to kill mucus-entrapped bacteria on a chronic basis, and the subsequent chronic inflammatory responses of the body to this type of surface infection, that lead to the syndromes of CB and CF. Therefore, enhancing the clearance of such thickened, adhered mucus from the airways is likely to benefit patients with these mucus-obstructive diseases.
The current afflicted population in the U.S. is 12,000,000 patients with the acquired (primarily from cigarette smoke exposure) form of chronic bronchitis and approximately 30,000 patients with the genetic form, cystic fibrosis. Approximately equal numbers of both populations are present in Europe. In Asia, there is little CF but the incidence of CB is high and, like the rest of the world, is increasing.
There is currently a large, unmet medical need for products that specifically treat CB and CF at the level of the basic defect that cause these diseases. The current therapies for chronic bronchitis and cystic fibrosis focus on treating the symptoms and/or the late effects of these diseases. Thus, for chronic bronchitis, 3-agonists, inhaled steroids, anti-cholinergic agents, and oral theophyllines and phosphodiesterase inhibitors are all in development. However, none of these drugs effectively treat the fundamental problem of the failure to clear mucus from the lung. Similarly, in cystic fibrosis, the same spectrum of pharmacologic agents is used. These strategies have been complemented by more recent strategies designed to clear the CF lung of the DNA (“Pulmozyme®”; Genentech) that has been deposited in the lung by neutrophils that have futilely attempted to kill the bacteria that grow in adherent mucus masses and through the use of inhaled antibiotics (“TOBI®”) designed to augment the lungs' own killing mechanisms to rid the adherent mucus plaques of bacteria. A general principle of the body is that if the initiating lesion is not treated, in this case mucus retention/obstruction, bacterial infections become chronic and increasingly refractory to antimicrobial therapy. Thus, a major unmet therapeutic need for both CB and CF lung diseases is an effective means of mobilizing airway mucus and promoting its clearance, with bacteria, from the lung.
In addition to CB and CF lung diseases, there is a large unmet need to facilitate the clearance of excess mucus secretions from the lungs in other mucoobstructive conditions. The overproduction of pulmonary mucus has been characterized in conditions including idiopathic pulmonary fibrosis, asthma, viral and bacterial lung infections, primary ciliary dyskinesia, and non-CF bronchiectasis, and mechanical lung ventilation. The accumulation of pulmonary mucus can harbor bacteria leading to chronic lung infections, as well as, reduces lung function. Thus, there is a need for therapeutic agents which facilitate the clearance of excess mucus.
Other mucosal surfaces in and on the body exhibit subtle differences in the normal physiology of the protective surface liquids on their surfaces, but the pathophysiology of disease reflects a common theme: an imbalance in the composition of the protective surface liquid and impaired mucus clearance. For example, in xerostomia (dry mouth) the oral cavity is depleted of liquid due to a failure of the parotid sublingual and submandibular glands to secrete liquid.
Similarly, keratoconjunctivitis sicca (dry eye) is caused insufficient tear volume resulting from the failure of lacrimal glands to secrete liquid or excessive evaporative fluid loss. In rhinosinusitis, there is an imbalance, as in CB, between mucin secretion, relative airway surface liquid depletion, and mucus stasis. Finally, in the gastrointestinal tract, failure to secrete Cl− (and liquid) in the proximal small intestine combined with increased Na+ (and liquid) absorption in the terminal ileum leads to the distal intestinal obstruction syndrome (DIOS). In older patients, excessive Na+ (and volume) absorption in the descending colon produces constipation and diverticulitis.
The high prevalence of both acute and chronic bronchitis indicates that this disease syndrome is a major health problem in the U.S. Despite significant advancements in the etiology of mucus obstructive diseases, pharmacotherapy of both CF and COPD have been characterized by an aging array of therapies, typically including inhaled steroids and bronchodilators for maintenance, and antibiotics and high-dose steroids for exacerbations. Clearly, drugs are needed that are more effective at restoring the clearance of mucus from the lungs of patients with CB/CF. The value of these new therapies will be reflected in improvements in the quality and duration of life for both the CF and the CB populations.
One approach to increase mucus clearance is to enhance the transportability of mucins via the disruption of the polymeric mucus structure. Mucin proteins are organized into high molecular weight polymers via the formation of covalent (disulfide) and non-covalent bonds. Disruption of the covalent bonds with reducing agents is a well-established method to reduce the viscoelastic properties of mucus in vitro and is predicted to minimize mucus adhesiveness and improve clearance in vivo. Reducing agents are well known to decrease mucus viscosity in vitro and commonly used as an aid to processing sputum samples (Hirsch, S. R., Zastrow, J. E., and Kory, R. C. Sputum liquefying agents: a comparative in vitro evaluation. J. Lab. Clin. Med. 1969. 74:346-353). Examples of reducing agents include sulfide containing molecules capable of reducing protein disulfide bonds including, but not limited to, N-acetyl cysteine, N-acystelyn, carbocysteine, cysteamine, glutathione, and thioredoxin containing proteins.

N-acetyl cysteine (NAC) is approved for use in conjunction with chest physiotherapy to loosen viscid or thickened airway mucus. Clinical studies evaluating the effects of oral or inhaled NAC in CF and COPD have reported improvements in the rheologic properties of mucus and trends toward improvements in lung function and decreases in pulmonary exacerbations (Duijvestijn Y C M and Brand P L P. Systematic review of N-acetylcysteine in cystic fibrosis. Acta Peadiatr 88: 38-41. 1999). However, the preponderance of clinical data suggests that NAC is at best a marginally effective therapeutic agent for treating airway mucus obstruction when administered orally or as an inhalation aerosol. A recent Cochrane review of the existing clinical literature on the use of NAC found no evidence to support the efficacy of NAC for CF (Tam J, Nash E F, Ratjien F, Tullis E, Stephenson A; Nebulized and oral thiol derivatives for pulmonary disease in cystic fibrosis. Cochrane Database Syst Rev. 2013; 12(7):CD007168.).
NAC, as a topical pulmonary therapeutic agent, is not optimal for the reduction of mucin disulfide bonds. Specifically, NAC does not possess the basic properties of an effective pulmonary drug as NAC (1) is a relatively inefficient reducing agent the airway surface environment (e.g., CF pH 6.5-7.2); and (2) is rapidly metabolized and cleared from the airway surface (Jayaraman S, Song Y, Vetrivel L, Shankar L, Verkman A S. Noninvasive in vivo fluorescence measurement of airway-surface liquid depth, salt concentration, and pH. J Clin Invest. 2001; 107(3):317-24). For example, in the pH environment of the airway surface (measured in the range of pH 6.0 to 7.2 in CF and COPD airways), NAC exists only partially in its reactive state as a negatively charge thiolate (Jayaraman S, Song Y, Vetrivel L, Shankar L, Verkman A S. Noninvasive in vivo fluorescence measurement of airway-surface liquid depth, salt concentration, and pH. J Clin Invest. 2001; 107(3):317-24). Furthermore, in animal studies, 14C-labeled NAC, administered by inhalation, exhibits rapid elimination from the lungs with a half-life of approximately 20 minutes (unpublished observation). The relatively low reducing activity at of NAC physiologic airway pH and the short half-life of NAC on the lung surface provide an explanation for the lack of strong clinical evidence for effective mucus reduction in mucus obstructive diseases.
Additionally, NAC is most commonly administered as a concentrated inhalation solution (Mucomysrt is a 20% or 1.27M solution). However, the administration of concentrated NAC solutions impact the tolerability of NAC as it exaggerates (1) the unpleasant sulfur taste/odor; and (2) pulmonary side effects including irritation and bronchoconstriction which can require co-administration of rescue medications such as bronchodilators. Although Mucomysta was approved by the FDA in 1963, no other reducing agents administered as an inhalation aerosol are currently available to treat muco-obstructive diseases. What are needed are effective, safe, and well-tolerated reducing agents for the treatment of diseases characterized by impaired mucus clearance.
As discussed above, compounds that could be useful in the treatment of muco-obstructive diseases as mucolytics often contain sulfides. These drugs, like NAC and DTT, typically have an unpleasant sulfurous odor/taste, can be oxidized (i.e., inactivated) easily, and are less tolerated. The addition of prodrug moieties could be a useful strategy to overcome these limitations to produce novel, well-tolerated therapeutics for a number of muco-obstructive diseases.